In the field of non-volatile memories, flash memory scaling beyond a 45 nm node has become a real issue. Technologies to face this challenge are ferroelectric, magnetic and phase change memories, the latter one being promising for the replacement of flash and showing characteristics that may allow replacement of other types of memories such as DRAM. Phase change memories are a possible solution for the unified memory being an important step in the electronics art. OTP (“on time programmable”) and MTP (“multiple times programmable”) memories open a field that may present a great opportunity for phase change memories as well.
Phase change memories are based on a reversible memory switching using, for instance, chalcogenide materials. The ability of these materials to undergo fast phase transition has led to the development of rewritable optical media (CD, DVD). The chalcogenide phase change materials may be divided in two classes, which are slightly different compositions, based on their crystallization mechanism. One class is the so-termed “nucleation dominated” material GeTe—Sb2Te3 tie line such as Ge2Sb2Te5, which may be used in ovonic unified memory (OUM) devices. In this concept, the phase change material may be in contact with a bottom-resistive electrode to switch reversibly to a small volume of phase change material. The other class is “fast growth material”, known in optical storage application (CD-RW/DVD+RW), which enables very fast switching (for instance 10 ns) with a proper phase stability.
Thus, phase change materials may be used for storing information. The operational principle of these materials is a change of phase. In a crystalline phase, the material structure is—and thus properties are—different from the properties in the amorphous phase.
The programming of a phase change material is based on the difference between the resistivity of the material and its amorphous and crystalline phase. To switch between both phases, an increase of the temperature is required. Very high temperatures with rapid cooling down will result in an amorphous phase, whereas a smaller increase in temperature or slower cooling down leads to a crystalline phase. Sensing the different resistances may be done with a small current that does not cause substantial heating.
The increase in temperature may be obtained by applying a pulse to the memory cell. A high current density caused by the pulse may lead to a local temperature increase. Depending on the duration and amplitude of the pulse, the resulting phase will be different. A fast cooling and large amplitude may quench the cell in an amorphous phase, whereas a slow cooling down and a smaller amplitude pulse may allow the material to crystallize. Larger pulse amplitudes, so-called RESET pulses, may amorphize the cells, whereas smaller pulse amplitudes will SET the cell to its crystalline state, these pulses are also called SET pulses.
WO 2005/093839 discloses an electric device having a resistor comprising a layer of a phase change material, which is changeable between a first phase with a first electrical resistance and a second phase with a second electrical resistance different from the first electrical resistance. The phase change material is a fast growth material. The electric device further comprises a switching signal generator for switching the resistor between at least three different electrical resistance values by changing a corresponding portion of the layer of the phase change material from the first phase to the second phase
However, when manufacturing conventional phase change material memory cells, it may be difficult to obtain a proper and reproducible quality of the manufactured memory cells.